Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser systems at the transmitter. These laser systems are designed or actively tuned to operate at different wavelengths. When their emissions are combined in an optical fiber, the resulting WDM optical signal has a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in semiconductor amplifier systems or gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman amplification scheme, although semiconductor optical amplifiers are also used in some situations. At the receiving end, the channels are usually separated from each other using, for example, thin film filter systems to thereby enable detection by separate detectors, such as photodiodes.
The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data in WDM systems is typically less than comparative non-multiplexed systems. This is because optical amplification systems required along the link is shared by all of the separate wavelength channels transmitted in the fiber. With non-multiplexed systems, each channel/fiber would require its own amplification system.
Nonetheless, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, optical components are required to combine the channels into, and separate the channels from, the WDM optical signal. Moreover, there is the danger of channel drift where the channels lose their spectral separation and overlap each other. This interferes with channel separation and demodulation at the receiving end.
Minimally, the optical signal generators, e.g., the semiconductor laser systems that generate each of the optical signals corresponding to the optical channels for a fiber link, must have some provision for wavelength control. Especially in systems with center-to-center wavelength channel spacings of less than 1 nanometer (nm), the optical signal generator must have a precisely controlled carrier wavelength. Any wander impairs the demodulation of the wandering signal at the far end receiver since the wavelength is now at a wavelength different than expected by the corresponding optical signal detector, and the wandering signal can impair the demodulation of spectrally adjacent channels when their spectrums overlap each other.
In addition to wavelength stability, optical signal generators that are tunable are also desirable for a number of reasons. First, from the standpoint of manufacturing, a single system can function as the generator for any of the multiple channel wavelength slots, rather than requiring different, channel slot-specific systems to be designed, manufactured, and inventoried for each of the hundreds of wavelength slots in a given WDM system. From the standpoint of the operator, it would be desirable to have the ability to receive some wavelength assignment, then have a generator produce the optical signal carrier signal into that channel assignment on-the-fly. Finally, in higher functionality systems such as wavelength add/drop devices, wavelength tunability is critical to facilitate dynamic wavelength routing, for example.
As tunable laser systems become more integrated and physically compact, a problem arises in that the spacing between the longitudinal laser cavity modes can be on the order of the proposed spectral spacings between channels. As a result, even if a given laser system can be tuned to have gain at the allocated channel wavelength, a longitudinal cavity mode for the laser cavity may not exist at that desired wavelength. In other words, two things must co-exist for a laser to generate light at the desired wavelength: 1) the cavity must have gain at that wavelength; and 2) the cavity must also have a longitudinal mode corresponding to the wavelength. Conventional lasers for WDM systems typically do not have provisions for tuning the cavity length to shift a cavity mode to the desired wavelength, actively.
In general, according to one aspect, the invention features a semiconductor tunable laser system. The system has a semiconductor chip, which functions as a laser gain medium within the laser cavity. A tunable Fabry-Perot filter is also provided within the laser cavity for selecting a wavelength of operation of the laser system. Finally, a cavity length modulator is included to modulate an optical length of the laser cavity at least over a distance corresponding to the spacings between the longitudinal modes of the laser cavity. Thus, according to the present invention, the tunable Fabry-Perot filter allows the laser cavity to have gain at the desired wavelength of operation, while the cavity length modulator tunes the laser cavity length such that a longitudinal cavity mode exists at the desired wavelength of operation.
According to the preferred embodiment, a controller is further provided that collectively tunes the Fabry-Perot laser to the desired wavelength of operation, and then controls the cavity length modulator so that a longitudinal mode of the cavity is controlled to reside at the desired wavelength of operation.
Further, in the preferred embodiment, the semiconductor chip comprises a semiconductor optical amplifier chip that functions as the laser gain medium. Further, according to the preferred embodiment, the tunable Fabry-Perot cavity comprises serial first and second partial reflectors and some means for changing an optical distance between the two reflectors. In the present implementation, an electrostatic deflection system is used whereby one of the reflectors is moveable and the controller modulates an electrical field between electrodes on the stationary and moveable reflectors to thereby change the distance between the stationary reflector and the movable reflector.
An electrostatic system is also preferably used to modulate the laser cavity length. In the present implementation, a movable, reflective membrane defines one end of the cavity. An electrostatic field controls the distance between the moveable membrane and a stationary electrode.
Preferably, the tunable laser system is integrated into a single hermetically sealed package. Specifically, the semiconductor chip, tunable Fabry-Perot filter, and the cavity length modulator are then attached or bonded to an optical bench, which is sealed within the package. As is common, a fiber pigtail enters the package via a fiber feed-through to connect and terminate above the bench to receive the laser beam from the laser cavity.
In the preferred embodiment, a wavelength locker system is further used. It has a differential wavelength filter and a multi-element detector. The controller then modulates the Fabry-Perot cavity to control the wavelength in response to the signal received from the multi-element detector.
In general, according to another aspect, the invention can also be characterized as a process for tuning a semiconductor laser system. This process comprises amplifying optical energy in a laser cavity with a semiconductor laser gain medium and then selecting a wavelength of operation of the laser cavity by tuning a Fabry-Perot filter within the laser cavity. Further, an optical length of the laser cavity is modulated over about the spacing between the longitudinal cavity modes. As described previously, the length can be tuned so that a cavity mode resides at the desired wavelength of operation.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.